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Well, you've got a point, I did not see clearly that /usr was
separated, still, IMHO, with current smallest disks being
hundreds of GBs big, leaving such a small amount for /
is looking for trouble, and as I assume we were talking
about a production server, well, that was my concern..

By my calculation, you have a TOTAL 13GiB of disk space. Not totally impossible, but if you plan any serious work, you definitely are going to want more.

Resizing after you've built your system is a HIGH RISK process. It's really easy to lose all you have.

/usr can easily need 20-30GiB
/var grows quickly -- the apt archives (.deb files) live there, and system logs can just keep growing. Again, I can't recommend less than 20GiB.
/home ? Well, I'm running out of headroom at 30GiB, but then I collect LOTS of e-mail. :-)